The present invention relates to a composition useful in hydrocarbon conversion processes. In one aspect, the invention relates to a catalyst containing a zeolite and lanthanum, which finds particular use in fluid catalytic cracking processes.
Catalytic cracking systems typically employ a fluidized bed or moving bed of finely divided particulate catalyst. The cracking catalyst is subjected to continuous cycling between cracking reaction and catalyst regeneration systems. In a fluidized catalytic cracking (FCC) system, a stream of hydrocarbon feed is contacted with fluidized catalyst particles in a reaction zone, usually at a temperature of 800-1100.degree. F., or higher. The reactions of hydrocarbons in the feed at the elevated conversion temperature result in deposition of carbonaceous coke on the catalyst particles and in the production of lower-molecular-weight hydrocarbons. After the processed hydrocarbons have been separated from the catalyst, the coked catalyst is stripped of volatiles and is passed to the catalyst regenerator. In the regenerator coked catalyst is contacted with an oxygen-containing gas and a desired amount of coke is burned off the catalyst particles, with heat evolved during the coke burnoff heating the catalyst particles. Heated and regenerated catalyst is returned to the reaction zone. The flue gas formed during coke burnoff is removed from the regenerator and passed into the atmosphere after appropriate treatment to remove solids, carbon monoxide, etc.
After regeneration, catalyst particles still contain some carbonaceous material. Removal of substantially all the coke from the catalyst during regeneration has been difficult when prior art systems have been used. Previously, when amorphous silica-alumina catalysts were used for catalytic cracking, there was little incentive to reduce the coke concentration in regenerated catalysts below about 0.2 weight percent. Most FCC catalysts now used, however, contain crystalline aluminosilicate zeolites, or molecular sieves. The zeolite-containing catalysts have usually been found to have higher activity and selectivity for the desired cracking reactions when the coke carbon content of the catalyst is as low as possible after regeneration. Thus, it is desirable to reduce the coke content of zeolite-containing catalysts as much as possible during regeneration. For this reason, increasing the degree of coke burnoff during regeneration has become an economic problem in the FCC processing art, the solution to which is presently being sought.
It is desirable to form CO.sub.2 as a combustion product during catalyst regeneration in FCC units, as opposed to forming CO. The formation of CO.sub.2 rather than CO increases the amount of heat released in the regenerator and reduces the amount of pollution control treatment needed before the regenerator flue gas can be passed into the atmosphere.
In addition to the degree of coke burnoff, the rate of coke burnoff in the catalyst regenerator is also important. A faster burnoff rate allows a smaller inventory of catalyst to be used in the regenerator and allows the regenerator to be sized smaller than would otherwise be possible.
It is known that the degree of coke burnoff and the rate of coke burnoff in an FCC catalyst regenerator can be increased by adding a promoter, such as a catalytic metal, to the FCC catalyst. Many promoter materials have been suggested in the art. Clearly, any promoter which is to be successfully used commercially must be extremely stable in order to withstand the continuous cycling between the high-temperature cracking reaction zone in the FCC system and the even higher-temperature regeneration zone. Several promoters which have been previously suggested have been found to be initially active in promoting a rapid coke burnoff rate and high degree of coke burnoff, but these promoters have been found to lose their effectiveness quite rapidly during operation under the severe conditions encountered in an FCC system. For example, one of the promoters found to be initially effective but to lose its effectiveness rapidly in commercial operations is chromia.
Netherlands patent application No. 72/03649 filed Mar. 17, 1972 discloses that cracking catalysts may be impregnated with rare earth metals to improve product yields and to reduce the amount of coke formed in the reaction zone. This application teaches that rare earth metals should be included in the catalyst in an amount about 1 to 5% greater than the amount which can be introduced into the catalyst by ion exchange with sodium or other cations in the zeolite component of the catalyst. The amount of rare earth metals utilized is calculated on the basis of RE.sub.2 O.sub.3. The application alleges that catalysts promoted with the rare earth metals acquire excellent selectivity, providing a relatively high yield of gasoline and reducing coke make.
Netherlands patent application No. 73/00884, filed Jan. 22, 1973 describes the addition of cerium or of rare earth metal mixtures containing a substantial amount of cerium to zeolite-containing FCC catalysts. This application describes the use of catalysts containing 5 to 15% of a crystalline aluminosilicate zeolite which has previously been exchanged with rare earth metals and which is dispersed in an amorphous silica-alumina matrix. The catalyst material is washed and is then treated with a dilute solution containing cerium cations or a mixture of rare earth metal cations with a substantial amount of cerium content, and is then filtered and calcined. The application describes the catalyst as having the cerium statistically dispersed in the catalyst composition and states that the cerium should be present in an amount of 0.05 to about 4%, calculated as cerium oxide in addition to ion exchanged cerium. The catalysts which have been promoted with cerium are described as being substantially equal in catalytic cracking activity to the similar catalysts which have not been cerium-promoted. The cerium-promoted catalysts are stated to have regeneration rates appreciably higher than untreated zeolite catalysts. The application theorizes that the cerium promoter functions to increase the coke and CO oxidation rates during regeneration. It states that the other rare earth metal cations do not seem to have any particular effect when utilized as FCC catalyst promoters.